PROJECT SUMMARY Dendritic cells (DCs) are the primary cell responsible for priming nave T cells, which is the first step in initiating T cell-dependent adaptive immunity, making them ideal targets for therapeutic modulation of immune responses. The short lifespan of DCs provides a powerful mechanism to regulate their activity and the anti- apoptotic and pro-apoptotic proteins BCL2 and BCL2L11 (also known as BIM) form a molecular clock to delimit activated DC lifespan. Defining transcriptional mechanisms controlling expression of these factors will identify novel pathways that could be targeted to tailor adaptive immunity through manipulation of DC lifespan. Long noncoding RNAs (lncRNAs) do not encode proteins but instead can function as effector molecules to regulate transcription. We recently discovered a conserved lncRNA that we named Morrbid (myeloid RNA repressor of BCL2L11 induced death) that controls granulocyte cell lifespan through the regulation of Bcl2l11 expression. Given the short half-life of DCs and the role of BCL2L11 in controlling DC survival, we predicted that Morrbid also controls DC lifespan. Indeed, Morrbid-/- DCs rapidly undergo apoptosis when activated. Scientific premise: Since deletion of Morrbid decreases DC survival and reduced DC lifespan impairs adaptive immunity we hypothesize that the lncRNA Morrbid is required for the survival of activated DCs to initiate robust T cell-mediated immune responses. Preliminary experiments in mice show that Morrbid-/- DCs are impaired in their ability to activate T cells in vivo. In aim 1 we will test whether the loss of Morrbid alters the survival of different lung DC subsets in vivo after intranasal delivery of lipopolysaccharide (LPS). We will test whether DC apoptosis caused by loss of Morrbid reduces the DC trafficking of fluorescent antigen to the mediastinal lymph node from the lung. Finally, using different DC activators and genetic rescue experiments, we will test whether increased apoptosis of Morrbid-/- DCs is dependent on BCL2L11. These experiments will determine if Morrbid regulates DC survival upon activation with different stimuli through regulation of BCL2L11 and will reveal if loss of Morrbid impairs antigen trafficking by DCs. In aim 2 we will test the requirement for DC Morrbid in adaptive immune responses. We will test the ability of Morrbid-/- bone marrow derived DCs to activate CD4 and CD8 TCR transgenic T cells in vitro and the ability of endogenous Morrbid-/- DCs to activate transgenic T cells in vivo. In mixed bone marrow chimeras, in which Morrbid deficiency is isolated to the DC compartment, we will test the requirement for Morrbid in initiating T cell and B cell responses in a model of allergic airway disease. These experiments will determine if Morrbid-/- DCs can process and present antigen, and will test whether they are impaired in their ability to initiate adaptive responses in vitro and in vivo. Impact: This work will elucidate mechanisms controlling DC survival, which could lead to the development of methodologies to manipulate DC lifespan and therefore adaptive immunity; extending DC lifespan could increase adaptive responses potentially boosting the efficacy of vaccination protocols and improving DC based cancer immunotherapies.